This application claims the priority of German Application No. 198 33 189.4, filed Jul. 23, 1998.
The invention relates to a steering system for a vehicle and a method for steering a vehicle.
Steering systems are known in which a steering operating device is mechanically uncoupled from the steering device exercising the force upon the wheels. Such an arrangement has, for example, the advantage that steering corrections, which become necessary because of criteria concerning the directional control and the driving dynamics, can be taken into account more easily and the steering performance can be correspondingly adapted to a respective driving situation. Furthermore, a less complicated construction is obtained, and the steering operating unit can be arranged in the vehicle in a more variable manner.
For example, steering systems are known, wherein a steering angle is tapped at the steering wheel and is set at the wheels by way of a control unit (compare German Patent Document DE 39 30 445 A1). In addition, the counterforce from the wheel-road effect affecting the wheels is determined and is fed back to the steering wheel so that, by way of this fed-back moment, the driver is provided with a vehicle handling sensation which corresponds to a mechanical handling sensation.
However, such steering systems have the disadvantage that steering vibrations may occur because of the static friction in the steering gear, particularly when the driver lets go of the steering wheel.
From European Patent Document EP 0 775 624 A2, a steering system is known, in which an operating moment can be tapped on a steering operating unit which is converted into a control current by a control unit. This control current, in turn, acts upon an actuator operating the steerable wheels. In addition, by means of a sensor, the steering angle of the wheel is determined. A corresponding signal is processed together with other signals by the control unit and a pertaining restoring moment is fed back to the operating unit. The vehicle driver operates the steering operating unit against this restoring moment.
It is an object of the invention to provide a simple steering system of the above-mentioned type which can effectively provide a good vehicle handling sensation without the occurrence of vibrations because of static friction in the steering system.
It is also an object of the invention to provide a method which provides a good vehicle handling sensation.
This object is achieved by a steering system with a steering operating device, an operating moment sensor which is assigned to the steering operating device and which senses the operation moment exercised on the steering operating device and generates a corresponding signal, a control unit which is constructed to process the signal generated by the operating moment sensor and emit an assigned desired steering moment or an assigned desired steering force, a steering device which is mechanically uncoupled from the steering operating device and which gets the desired steering moment or the desired steering force at the steering wheels, a steering angle sensor which senses a steering angle corresponding to the alignment of the steerable wheels and supplies a corresponding signal to the control unit which, in turn, determines a pertaining desired angle of the steering operating device and a feedback unit which sets the determined desired angle at the steering operating device.
The object is also achieved by a method for steering a vehicle, comprising the acts of sensing a moment or force being exerted on a steering operating device, processing the operating moment by the control unit and generating an assigned desired steering moment or steering force, setting the assigned desired steering moment or steering force at the steerable wheels, sensing a steering angle corresponding to the alignment of the steerable wheels, determining a pertaining desired angle of the steering operating device by the control unit based upon the sensed steering angle and setting the pertaining desired angle of the steering operating device.
Accordingly, the steering system for a vehicle according to the invention is constructed such that, in principle, a moment input is determined on a steering operating element and a steering angle is fed back to it. A steering moment computed by the control unit is fed to the wheels. The wheels will then automatically adjust such that this moment and the countermoment originating from the wheel-road effect compensate one another (balancing of moments). In this case, the desired steering moment defined by the control unit corresponds to the moment which is generated by the lateral wheel force.
In this context, it is particularly pointed out that the terms xe2x80x9csteering momentsxe2x80x9d and xe2x80x9csteering forcesxe2x80x9d are to have the same formal meaning.
In a feedback loop, the actually occurring (equilibrium) steering angle of the steerable wheels is determined first and is transmitted to the control unit. This control unit computes a pertaining desired angle at the steering operating unit, which is set by way of a feedback unit.
On the whole, therefore, a defined moment is set at the wheels in a first branch of the control. In the second branch of the control, the detected steering angle is transmitted back to the steering operating unit.
This simple control defines a steering system which provides an effective and good vehicle handling sensation, without any vibration effects due to static friction in the steering system.
According to an advantageous embodiment of the steering system, the moment is set at the wheels in an automatic control operation (separate control loop). For this purpose, a moment sensor senses the actual steering moment currently existing at the wheel and compares it with the desired steering moment. If a deviation between the two values is determined, the steering device will operate the wheels such that the actual steering moment will correspond to the required desired steering moment.
Analogously, a preferred embodiment of the invention is characterized such that, in a further automatic control (another separate control loop), the angle is set at the steering operating unit. For this purpose, an angle of rotation sensor detects the actual angle of rotation existing at the steering operating device and compares it with the desired angle computed by the control unit. If a deviation is determined between the two values, the steering operating device is controlled such that the actual angle of rotation corresponds to the desired angle.
The actuating devices provided for operating the wheel or the steering operating device may have an electric, hydraulic or pneumatic or other known construction.
Since the steering systems are generally safety-relevant to a high degree, preferably all hardware and software elements included in the steering system are provided in a redundant manner.
The steerable wheels may be connected with one another by way of a wheel linkage. As an alternative, each wheel can be separately steerable, e.g. by actuators assigned to each steerable wheel, respectively.
According to a preferred embodiment of the invention, the connection between the steering operating device and the actuator for adjusting the desired angle at the steering operating device has an elastic design; for example, by means of a torsion spring. The elastic design substitutes for an elasticity which is incorporated in a conventionally mechanically coupled steering system by way of various mechanical components and connections.
Of course, one could also compute the desired steering moment or the desired angle such that a virtual elasticity or stiffness is created between the vehicle driver""s input and the wheel-road effect. This measure improves comfort and vehicle handling and increases the directional control.
An advantage of a mechanical uncoupling of the steering operating device and the steering device consists of the simple compensation of various influences concerning the directional control and the driving dynamics. For this purpose, the control unit can be constructed such that additional parameters, such as the vehicle speed, the yaw rate, the slip angle, the road condition, the lateral acceleration or other characteristic data, are analyzed and taken into account when forming the desired steering moments and/or the desired angles.